1. Field of the Invention
The instant invention relates to fuels based on methanol and/or ethanol which contain additionally mixtures of C.sub.4 -hydrocarbons with C.sub.5 /C.sub.6 -and/or C.sub.5 -C.sub.7 -hydrocarbons and/or gasoline.
2. Description of the Prior Art
Distilled ("pure") methanol has been extensively investigated as an alternate fuel for a number of years (Chemische Technologie, Winnacker-Kuchler, Vol. 5, Organische Technologie I, 4. Ed. 1981, p. 517).
Higher alcohols and water as blending-components for methanol and the use of these blends as fuels are known (N. Iwai, The combustion of methanol mixed with water, Second Nato-Symposium; Nov. 4-8, 1974, Dusseldorf).
In U.S. Pat. No. 2,365,009 combinations of alcohols with 1 to 5 C-atoms with saturated and unsaturated hydrocarbons with 3-5 C-atoms as fuels are described.
The same applicant discloses in Continuation in part U.S. Pat. No. 2,404,094, fuels, which either consist of absolutely pure methanol or commercial, refined methanol, which is free of water (column 6, lines 8-11) in combination with aliphatic C.sub.3 -C.sub.5 hydrocarbons. This Patent also claims a methanol fuel, which contains 2-20% of an aliphatic C.sub.4 - or C.sub.5 -hydrocarbon. According to column 5, lines 22-27, hydrocarbons in highly purified form are preferred. Furthermore the hydrocarbons used, may be partly unsaturated (column 5, lines 28-34). The examples disclose (table 1) as added hydrocarbons n-pentane, iso-pentane and a C.sub.4 -cut, which contains up to 20% of butenes. According to claims 5 and 6 a mixture of saturated C.sub.5 -hydrocarbons can be used for the special case of a fuel for the cold start of aviation engines.
In U.S. Pat. No. 2,365,009 also mixtures of ethanol with aliphatic C.sub.3 -C.sub.5 -hydrocarbons are described, whereby the aliphatic hydrocarbons may be either saturated or unsaturated and whereby isopentane (claim 8 and table 1) is preferred. Further disclosures of ethanol/hydrocarbon mixtures are DE-OS 28 06 673 and DE-OS 32 11 775.
It is well known to the artisan that in certain countries, with a high supply of ethanol, for example in Brazil, ethanol is used in pure form as well as in mixtures with gasoline as a fuel (Chemical Engineering Progress, April 1979, page 11).
On the other hand it is also known that the lower alcohols have specific disadvantages with regard to their use as fuel, for example bad cold start behavior, bad driving behavior at low outside temperature, unsatisfactory mixing in particular with hydrocarbons at low temperatures and the broad explosibility limits.
The cause of the cold-start problems, in particular at low temperatures, is to be sought in the low ignitibility of methanol and ethanol.
A measure for the ignitibility is the vapor pressure of a motor-fuel, which is determined by the so-called Reid-Test at 37,7.degree. C. Methanol for example has a Reid vapor pressure (RVP) of 350 mbar, whereas gasoline has a Reid vapor pressure of 700 mbar.
At an outside temperature below 15.degree. C. the vapor pressures of methanol and ethanol are too low for the formation of ignitible gasphase mixtures.
The explosibility limits for pure (distilled) methanol in air are 6,75 to 36,7 percent by volume resulting in an explosive mixture of fuel and air in the gasphase in the fuel tank between +15.degree. and +25.degree. C. Addition of 6 to 9 percent by weight of isopentane reduces the upper explosibility limits to -7.degree. C. at summer temperature and -20.degree. C. at winter temperature. As a result the safety problems are essentially avoided. In the state of the art isopentane has been considered therefore and because of its excellent solubility in methanol as well as in ethanol in particular at low temperatures as the preferred additive (isopentane is 2-methylbutane).
The most favorable adjustment of the vapor pressure of neat methanol has turned out to be 700 mbar for summer- and 900 mbar for winter-fuel (which are the upper vapor pressure values of the German fuel DIN-Norm 51600).
In consideration of the problems described above and in consideration of the state of the art and as a consequence of continued investigations, isopentane (2-methylbutane) has been hitherto selected as the optimum hydrocarbon additive.
The fuel consisting of distilled (refined) methanol and isopentane, which is known as M 100 fuel has been tested in extensive field tests for several years, in particular in municipal car fleets in the Federal Republic of Germany (s. H. Muller, 27th DGMK-Conference, 6th to 8th of October, 1982).
Although methanol, which contains isopentene, meets to a certain extend the expectations with regard to a reliable motor-fuel, the tests have unexpectedly shown that there still remain substantial disadvantages.
In particular during operation at summer temperatures the vapor pressure by using methanol/isopentane mixtures is to high even after reduction of the isopentane-content to 5-6 weight percent, accompanied by undesirable degassing of isopentane. Whereas at winter temperatures despite an isopentane content of up to 9 percent by weight a decrease of the vapor pressure is observed which leads to bad cold start behavior at temperatures below -10.degree. C.
Object of the present invention therefore was to make fuels available, based on methanol and ethanol, resulting in an improved driving behavior in particular at relatively high and relatively low outside temperatures, as required in practical operation of a motor-vehicle and which would simultaneously permit safe operation, avoiding the formation of an explosive mixture inside of the fuel tank.